A number of serious medical conditions may be treated in a minimally invasive manner with various kinds of catheters designed to reach treatment sites internal to a patient's body. One such medical condition is atrial fibrillation—a serious medical condition that results from abnormal electrical activity within the heart. This abnormal electrical activity may originate from various focal centers of the heart and generally decreases the efficiency with which the heart pumps blood. It is believed that some of these focal centers reside in the pulmonary veins of the left atrium. It is further believed that atrial fibrillation can be reduced or controlled by structurally altering or ablating the tissue at or near the focal centers of the abnormal electrical activity.
Cryotherapy, or the cooling of body tissue, is one method of ablating tissue of the heart and pulmonary veins to control atrial fibrillation. Cryotherapy may be delivered to appropriate treatment sites inside a patient's heart and circulatory system by a cryotherapy catheter. A cryotherapy catheter generally includes a treatment member at its distal end, such as an expandable balloon having a cooling chamber inside. A cryotherapy agent may be provided by a source external to the patient at the proximal end of the cryotherapy catheter and delivered distally through a lumen in an elongate member to the cooling chamber where it is released. Release of the cryotherapy agent into the chamber cools the chamber, and hence the balloon's outer surface that is put in contact with tissue to perform ablation. The cryotherapy agent may be exhausted proximally through an exhaust lumen in the elongate member to a reservoir external to the patient.
During a cooling cycle, there is generally a constant circulation of the cryotherapy agent to and from the distal part of the catheter, and it is important that the path of circulation not be disrupted. Any disruption in this path of circulation may present significant risks to the patient undergoing treatment. For example, disruptions in the circulation path may lead to loss of control of the cooling process, which may cause non-targeted tissue to be ablated. As another example, disruptions in the circulation path may lead to loss of control of pressure associated with a liquid-gas phase change in some cryotherapy agents inside the cooling chamber, which may result in uncontrolled expansion or rupture of portions of the catheter inside the patient.
The circulation path for the cryotherapy agent generally includes two separate lumens within an elongate member. One lumen fluidly couples the external cryotherapy source to the chamber of the balloon, and the other lumen is used for exhaust and fluidly couples the chamber of the balloon with the external reservoir. To maintain a circulation path, these lumens must be protected against being “pinched off,” especially as the catheter is navigated through various turns—some of which may be sharp turns—in the vessels and organs of a patient's body.